1. Technical Field
The present invention relates to the electrochemical production of peroxo-disulfuric acid with the use of diamond-coated electrodes.
2. Related Art
With a normal potential (Eo) of 2.01 V, peroxo-disulfuric acid (H2S2O8) is one of the strongest known oxidizing agents, which is used in a wide variety of fields. The most important areas of application of peroxo-disulfuric acid include etching processes in the electronics industry and the production of particular plastics, for example use in the polymerization of acrylonitrile.
Peroxo-disulfuric acid also has applications in waste treatment, the oxidation of dyestuffs and the bleaching of fibers. In addition to this, peroxo-disulfuric acid is an important intermediate product for the electrochemical production of hydrogen peroxide.
The mechanism of the formation of peroxo-disulfuric acid by the anodic oxidation of sulfuric acid is complex. It is assumed that it comprises the formation of hydroxyl radicals. According to the mechanism water is first of all discharged at the anode with the formation of adsorbed hydroxyl radicals (see Equation 1). The hydroxyl radicals, which are present adsorbed at the diamond surface, react with the hydrogen sulfate ions (see Equation 2) contained in the electrolyte, which form the actual peroxo-disulfuric acid in a subsequent dimerization step (see Equation 3).
 H2O→OH.+H++ε−  (1)HΣO4−→HSO4.+e−  (2)(2 HSO4.→H2S2O8).  (3).
High concentrations of sulfuric acid and high current densities are required in this case, because when dilute solutions and small current densities are present, the low concentration of discharged sulfate ions results in the latter not reacting with one another (see Equation 3), but with the water, with the formation of oxygen:SO4+H2O→H2SO4+½O2  (4)
There may furthermore be formed as by-products: oxygen by the decomposition of water, ozone, peroxo-monosulfuric acid and hydrogen peroxide, according to the following (see Equations 5 and 6:H2S2O8+H2O→H2SO5+H2SO4  (5)H2SO5+H2O→H2SO4+H2O2  (6).
The effectiveness of the electrochemical peroxo-disulfuric acid production depends substantially on the electrode material used, of which high requirements are made because of the prevailing oxidative and corrosive conditions.
For example, the electrode material must be corrosion-resistant and stable against anodic dissolution.
Furthermore, the peroxo-disulfuric acid formation takes place in a potential range in which water is already decomposed with the production of oxygen. In order to suppress the competing oxygen production, therefore, the electrode material must exhibit a high overvoltage for the reaction.
Flat, large-area platinum electrodes are currently used for the large-scale electrochemical production of peroxo-disulfuric acid, with high sulfuric acid concentrations and high current densities. However, the platinum electrodes are gradually dissolved in the course of the reaction, so that the corrosion products obtained have to be removed by complicated cyclical means.
In order to obtain a satisfactory yield, a highly-concentrated sulfuric acid solution containing 7.5 moles must furthermore be used as electrolyte. Highly-concentrated sulfuric acid solutions of this kind may now, however, because of the oxidative and corrosive properties, be handled in special units and are therefore expensive in equipment terms.
Because of the expensive equipment required, peroxo-disulfuric acid is produced in plants specially equipped for it and has to be procured from there. It would be desirable, however, for peroxo-disulfuric acid to be able to be produced as required directly on site, that is to say at the place of use, since peroxo-disulfuric acid is because of its extremely reactive properties difficult to store and in addition free peroxo-disulfuric acid is subject to rapid hydrolysis in aqueous solution.
Just recently, diamond-coated electrodes have because of their high chemical stability been attracting increasing interest for applications in electrochemical processes.
Such electrodes, in which a boron- or nitrogen-doped diamond layer is applied to a suitable support material, may be obtained in the main by means of the known CVD (Chemical Vapor Deposition) technique.
For example, EP 0 714 997 B1 discloses the use of an electrode of a metal-containing substrate, in particular titanium, to which a boron-doped diamond layer has been applied, for the oxidation of spent photographic baths and in the electronics or optoelectronics sectors.
It has been found, however, that the adhesive strength of diamond layers on metal-containing support materials such as titanium is not satisfactory.
To improve the adhesiveness, therefore, in EP 0 730 043 A1 an intermediate layer is provided between the support material and the diamond layer, which consists of the decomposition products of a metallocene, preferably biscyclo-pentadienyltitanium chloride.
The production of diamond-coated electrodes with silicon as support material for small areas of not more than 1 cm2 is described for example by G. M. Swain in: Adv. Mater. 6 (1994), p.388.
In a number of papers, moreover, a very great potential range is reported for diamond electrodes, in which no water decomposition and hence oxygen production occurs (H. B. Martin, A. Argoitia, U. Landau, A. B. Anderson, J. C. Angus: J. Electrochem. Soc. 143 (1996) L 133; F. Beck, H. Krohn, W. Kaiser, M. Fryda, C. P. Klages, L. Schäfer: Electrochimica Acta 44 (1998) 525).
However, there occurs also there, in the potential ranges favorable for the electrochemical production of peroxo-disulfuric acid, a significant evolution of oxygen, so that a suitability in principle of the electrodes for a peroxo-sulfuric acid production in large, economically significant yields-specifically also with low sulfuric acid concentrations-was not able to be assumed.
It is also reported that ozone may be obtained with diamond electrodes having silicon as support, (A. Perret, W. Haenni, P. Niedermann, N. Skinner, Ch. Comninellis, D. Gandani: Electrochemical Society Proceedings, Volume 97-32 (1997) 275).
The diamond-coated electrodes described above exhibit in general the disadvantage that either the diamond layer may be deposited only on small surfaces (G. M. Swain op. cit.) or, as disclosed in EP 0 730 043 A1, electrochemically stable electrodes with sufficiently firmly adhering diamond layers may be obtained only with the use of a specially applied intermediate layer.
The object of the present invention is to provide a process for the electrochemical production of peroxo-disulfuric acid and peroxo-disulfates, with which peroxo-disulfuric acid or the peroxo-disulfate may be obtained in economically significant yields on a large scale, including with low sulfuric acid concentrations.
The object according to the invention is achieved by a process in which peroxo-disulfuric acid and peroxo-disulfates are produced electrochemically with the use of electrodes coated with doped diamond.
Surprisingly, it was found according to the invention that contrary to expectations electrodes coated with doped diamond are extremely well suited to the electrochemical production of peroxo-disulfuric acid or peroxo-disulfates.
Below the term “peroxo-disulfuric acid” will be used collectively for the compounds peroxo-disulfuric acid and peroxo-disulfates produced.
In particular, if such electrodes are used, sufficiently high yields of peroxo-disulfuric acid may still be obtained even with low sulfuric acid concentrations. The finding is completely contrary to the prevailing view, according to which a high sulfate ion concentration is essential for obtaining high yields and avoiding subsidiary reactions.
Below the electrodes coated with doped diamond will also be referred to by the shortened form “doped diamond electrodes”.